1. Field of the Invention
The present invention relates to an object (a product including a machine, a manufacture, and a composition of matter) and a method (a process including a simple method and a production method). In particular, one embodiment of the present invention relates to a power storage device, a power storage system, a semiconductor device, a display device, a light-emitting device, and an electrical appliance, and further to a manufacturing method thereof and a driving method thereof. More specifically, one embodiment of the present invention relates to a power storage device, a power storage system, a semiconductor device, a display device, a light-emitting device, and an electrical appliance which include an oxide semiconductor, and further to a manufacturing method thereof and a driving method thereof. In particular, one embodiment of the present invention relates to a power storage device and a charging method thereof.
2. Description of the Related Art
In recent years, various power storage devices such as non-aqueous secondary batteries including lithium-ion secondary batteries and the like, lithium-ion capacitors, and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electrical appliances, for example, portable information terminals such as mobile phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today's information society.
A lithium-ion secondary battery, which is one of non-aqueous secondary batteries and widely used due to its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material such as graphite capable of occlusion and release of lithium ions, a non-aqueous electrolyte solution in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate, and the like. A lithium-ion secondary battery is charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the non-aqueous electrolyte solution and inserted into or extracted from the active materials of the positive electrode and the negative electrode.
The capacity of such a lithium-ion secondary battery or the like is determined by the amount of lithium inserted and extracted into/from the positive electrode. On the other hand, since decomposition reaction of the electrolyte solution occurs at the negative electrode, lithium is used in formation of a film called a solid electrolyte interphase (SEI), which may lead to a decrease in the capacity of the battery.
If decomposition reaction of the electrolyte solution similar to that at the negative electrode occurs also at the positive electrode, the decomposition reaction can cancel out the decomposition reaction at the negative electrode. However, since the potential of the positive electrode is not sufficiently higher than the oxidation potential of the electrolyte solution, the amount of reduction reaction at the negative electrode is larger than the amount of oxidation reaction at the positive electrode.